Physical Sciences: Year In Review 2011

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This article was originally published in the Britannica Book of the Year, an annual print publication that
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Scientists made advances in the study of hydrogen bonds, the use of sunlight and water for generating hydrogen gas, and the detection of water pollutants such as petroleum. Physicists created long-lasting antihydrogen, induced a cell to shine laser light, and may have observed faster-than-light neutrinos. Astronomers discovered a new moon of Pluto and many interesting extrasolar planets. The American space shuttle program ended after 30 years, and China launched its first space station module.

Physical Chemistry

Two studies reported in 2011 concerned advances in the understanding of hydrogen bonding. Hydrogen bonds involve highly changeable interactions that are individually very weak but that in bulk are of fundamental importance to a wide range of phenomena, from the workings of climate to the formation of DNA molecules. Since hydrogen atoms have a very small mass, they tend to behave according to quantum-mechanical rules rather than by Newtonian rules governing larger masses. Using computational techniques, Angelos Michaelides of University College, London, and co-workers found that these quantum-mechanical effects weaken already weak hydrogen bonds but add strength to stronger ones. The scientists expected that this discovery would lead to more precise calculations of the strength and other characteristics of hydrogen bonds in any given hydrogen compound and therefore allow chemists to predict more precisely the behaviour of hydrogen-containing bulk materials such as water.

The second study dealt with the discovery that the hydrogen bonds of water molecules at the boundary between a body of water and air are not strictly liquid or gaseous. The nature of this interface between water and air, which exists over more than 70% of Earth’s surface, affects the atmosphere and the environment. Alexander Benderskii from the University of Southern California and colleagues found that the interface is only one molecule thick and that water molecules at this boundary have one of their two hydrogen atoms in the water and the other in the air. The hydrogen atom in the air acts as if it is in the gas phase, whereas the hydrogen atom below, in the water, acts as if it is in the liquid phase. This is important because many of the properties of water are determined by how the hydrogen atoms in its molecules bond chemically. The bonding of a molecule that straddles the water’s surface is slightly weaker in comparison with the hydrogen bonds of molecules deeper in the bulk section of water and is similar to that of free water molecules in the air. The molecules at the surface change in and out constantly and are sometimes in the liquid phase, sometimes at the surface, and sometimes in the gas phase. As a result, there is a very fast transition of water molecules from gas-phase to liquid-phase behaviour and vice versa.

Cheap, efficient solar-energy conversion has long been a goal of scientists. In 2011 Daniel Nocera from MIT and colleagues reported on the development of a device that uses sunlight and water to produce hydrogen gas. The apparatus, about the size of a playing card, incorporated a silicon solar cell coated with catalytic materials. In one version of the device, one side of the solar cell was coated with a cobalt-based catalyst on a thin protective layer of indium tin oxide and the other side was coated with an alloy of nickel, molybdenum, and zinc. When the device is placed in water and illuminated by sunlight, the solar cell absorbs light and produces electrical energy that promotes chemical reactions between its coated surfaces and the water, effectively splitting water into its component elements, hydrogen and oxygen. In the version described, oxygen bubbles from the side of the solar cell coated with the cobalt-based catalyst and hydrogen from the other side. The hydrogen gas generated can then be collected and used as a fuel (in a fuel cell, for example). The device represented an advance over related technology by not requiring relatively rare chemical elements and for its ability to operate in ordinary fresh water and seawater. The resulting “artificial leaf” was inexpensive to produce, and the researchers envisioned the device’s being used in poor countries that do not have easy access to large amounts of energy.

Organic Chemistry

In the 1950s Stanley Miller from the University of Chicago conducted a set of now-famous experiments to probe the origins of life on Earth. These experiments involved sending an electric charge, meant to simulate lightning, through a chamber filled with gasses thought to have formed the early atmosphere and then determining whether chemical precursors of life had been produced in the chamber. Miller followed up his published results with additional experiments in which he varied the composition of the gasses in the chamber. Miller, who died in 2007, left unanalyzed samples from these experiments with a former student, Jeffrey Bada. Bada, of the Scripps Institution of Oceanography, La Jolla, Calif., and colleagues used modern techniques such as high-performance liquid chromatography and time-of-flight mass spectrometry to study the preserved samples. These modern techniques, which were many times more sensitive than those used in the 1950s, detected a total of 23 amino acids, including 7 organosulfur compounds in a sample that Miller had produced by using a gaseous mixture of hydrogen sulfide, methane, ammonia, and carbon dioxide. Hydrogen sulfide was not included in Miller’s earlier experiments, but he later used the compound because, according to some scenarios, it may have entered the early terrestrial environment in the plumes of volcanic eruptions. The samples, which had been stored in vials for more than 50 years, had roughly equal proportions of left-handed and right-handed varieties of many of the amino acids. This finding was an indication that the amino acids analyzed were generated by the experiment and not introduced later accidentally, since all living organisms produce only left-handed amino acids. The experiment was the first to show that sulfur-containing amino acids, vital to life, can be produced from a spark-discharge experiment. In addition, the overall quantities of the amino acids that were analyzed were comparable to those found in a type of meteorite rich in carbon, perhaps signifying that hydrogen sulfide may have had a key role in the environments of the early solar system.

Environmental chemists made advances during the year in the detection of water pollutants. Sang-Eun Oh and researchers from Arizona State University and Kangwon (S. Kor.) National University developed a system that uses Acidithiobacillus, a genus of sulfur-oxidizing bacteria found in wastewater, to monitor the overall toxicity of the water. Scientists usually monitored water pollution by looking at changes in the activity of specific microorganisms. This approach, however, was generally expensive and slow and could detect only certain pollutants. In the presence of oxygen and water, the Acidithiobacillus bacteria digest elemental sulfur and convert it to sulfate salts and protons. When nitrates, perchlorates, dichromates, or other pollutants are present, the bacteria’s conversion of sulfur is slowed, reducing the yield of sulfate and protons. As a result, the pH of the water rises and the water’s electrical conductivity decreases. These properties can be easily monitored, and the system can be used to measure water pollution as low as 5 to 50 parts per billion. The bacteria-based biosensor can detect toxicity within minutes to hours, which may be fast enough for use as an early-warning system to circumvent environmental pollution and threats to public health.

As known from such major oil spills as the Deepwater Horizon oil spill in 2010, petroleum can be a very serious water pollutant. To detect the amount of petroleum in water, oil workers use both ultraviolet fluorescence and infrared spectroscopy. Researchers from the University of Liverpool, Eng., used a technique that can measure oil levels in seawater more precisely and at lower concentrations than these methods. Stephen Taylor and colleagues tested the technique, called membrane inlet mass spectrometry, in the harsh conditions of the North Sea off Scotland. This type of mass spectrometry uses a membrane to keep water and salt from entering the instrument while letting through molecules of petroleum and other organic substances. It enabled the scientists not only to detect low levels of pollutants but also to identify what types of toxic hydrocarbons, such as benzene, toluene, and xylene, were in the oil. Use of this technique would allow oil workers to identify problems in the oil-extraction process and correct them more easily. The research conducted by Taylor and co-workers represented the first time that the technology had been used out in the field in harsh sea conditions. They were able to detect oil concentrations as low as 15 mg per litre, one-half the legal oil-discharge limit in the United Kingdom and, on the basis of the hydrocarbons found in the samples, were able even to distinguish contamination from two types of petroleum.

Industrial Chemistry

Renewable-energy production presents a number of challenges that need to be addressed for successful large-scale commercialization. It requires economical energy storage that can hold high levels of energy over extended electrical cycling (charging and discharging). By depositing manganese dioxide on porous textiles coated with an atom-thick layer of graphene, Zhenan Bao and colleagues at Stanford University created a flexible electrode material for high-performance capacitors. They placed the electrode in a solution of sodium sulfate together with a second electrode made from a textile coated with carbon nanotubes to create a supercapacitor with a maximum power density of 110 kW/kg that could hold 95% of its energy through 5,000 recharging cycles. The key to its high-energy load was the large surface area of the electrodes, and the scale of the reaction could be easily increased to industrial levels, making the technique easily transferable to large-scale energy production.

Physics

Particle Physics

In 2011 technological developments enabled physicists to close in on the answers to outstanding problems in the physics of fundamental particles. One such problem concerns antimatter, the mirror image of normal matter; when matter and antimatter interact, they annihilate each other. Antihydrogen, consisting of a positively charged electron (or positron) and a negatively charged nucleus, was detected for the first time in 1995. The ALPHA international consortium at CERN, near Geneva, succeeded in producing and storing antihydrogen for up to 17 minutes, making it possible to compare its properties with those of normal hydrogen. Any observed differences might suggest a solution to the problem of the vast preponderance of normal matter over antimatter in the universe. On the other hand, they would also contradict the standard model of fundamental particle physics, which assumes identical properties for matter and antimatter. In a different approach, Masaki Hori and co-workers at the Max Planck Institute, Garching, Ger., examined a molecule made up of a helium atom and an antiproton, giving a relative mass of the electron and antiproton agreeing with that of the electron and proton. The STAR collaboration at the Relativistic Heavy Ion Collider, Brookhaven National Laboratory, Upton, N.Y., produced the first antihelium nuclei, which may also provide a test of matter-antimatter asymmetry.

Fundamental particle theory was tested in another way. One popular extension of the standard model is the theory of supersymmetry, which postulates that each particle has a heavier “supersymmetrical” partner that rarely interacts with normal matter. However, first results from the Large Hadron Collider at CERN produced no evidence of such particles.

Condensed State

The study of graphene—a two-dimensional lattice of carbon atoms on an insulating substrate—produced results that may lead to a new generation of electronic devices, since electrons can travel in graphene 100 times faster than in silicon. Yanqing Wu and co-workers at the IBM Thomas J. Watson Research Center, Yorktown Heights, N.Y., studied graphene transistors that had cut-off frequencies as high as 155 GHz and that, unlike conventional devices, worked well at temperatures as low as 4.3 K (−268.9 °C, or −451.9 °F). Ming Liu and colleagues at the NSF Nanoscale Science and Engineering Center, Berkeley, Calif., demonstrated a high-speed broadband electro-optical modulator with high efficiency and an active device area of only 25 μm2. Such a device could lead to new designs of optical communications on chips. Vinay Gupta and colleagues from the National Physical Laboratory, New Delhi, made luminescent graphene quantum dots blended with organic polymers for use in solar cells and light-emitting diodes, which could offer better performance at lower cost than other polymer-based organic materials. By combining graphene with extremely small metal wires called plasmonic nanostructures, T.J. Echtermeyer, of the University of Cambridge, and co-workers made graphene-based photodetectors that were 20 times more efficient than those made in previous experiments.

Other two-dimensional systems were studied. A.F. Santander-Syro’s group at Université de Paris-Sud, Orsay, France, showed that there was a two-dimensional electron gas at the surface of the material SrTiO3.

One possible way for future computers to store information would be to encode data in the spin of electrons; such a computer has been called “spintronic.” Kuntal Roy and colleagues at Virginia Commonwealth University made a great step to producing a spintronic device by making a small spintronic switch in which very small amounts of energy would cause a piezoelectric material to move and thus change the spins of electrons in a thin magnetic layer. Devices using such switches could be powered by only very slight movements.

Optics and Lasers

Two new types of laser appeared in 2011. Yao Xiao and colleagues at the department of optical instrumentation, Zhejiang University, Hangzhou, China, reported lasing action at 738 nm (nanometres), using a folded wire 200 nm in diameter. The configuration made possible a tunable single-mode nanowire laser. Malte Gather and Seok-Hyun Yun at Harvard Medical School created a “living laser” by using biological material. Green fluorescent protein that had been inserted into human embryo kidney cells was used in a tiny optical cavity to produce laser light. This technique could be used to study processes in a living cell.

A cell that is genetically designed to produce the light-emitting molecule, green flourescent …

Gather/Yun

In a different region of the electromagnetic spectrum, J.R. Hird, C.G. Camara, and S.J. Putterman at the department of physics and astronomy, University of California, Los Angeles, investigated the triboelectric effect, in which electric currents are generated by friction. When the team pulled apart silicon and a metal-coated epoxy, a current generated by the friction was found to produce a beam of X-rays. This method could lead to a new generation of simple and cheap sources for X-ray imaging.

Lasers and optical devices for high-speed communications and information processing were being studied in many laboratories, with an emphasis on efficiency and reproducibility. Bryan Ellis and co-workers at Stanford University developed an electrically pumped quantum dot laser that produced continuous wave operation with the lowest current threshold yet observed. Matthew T. Rakher and colleagues at National Institute of Standards and Technology, Gaithersburg, Md., devised a system for simultaneous wavelength translation and amplitude modulation for single photons, using the “blending” in a crystal of photons from two separate laser sources. Georgios Ctistis and colleagues at the University of Twente, Enschede, Neth., built a switch that changed state in just one-trillionth of a second (10−12 s).

Quantum Information

Quantum information systems involve photons that are “entangled”—perfectly correlated over long distances. For storage and transmission of such photons, practical quantum memories are required for storing and recalling quantum states on demand with high efficiency and low noise. For transmission occurring over long distances, memory repeaters are required for receiving input data and retransmitting.

In 2011 a number of groups demonstrated designs for such devices. M. Hosseini and colleagues at the Australian National University in Canberra reconstructed quantum states that had been stored in the ground states of rubidium vapour with up to 98% fidelity. Christoph Clausen and co-workers at the University of Geneva demonstrated entanglement between a photon and a physical system. One photon from an entangled pair was stored in a Nd:Y2SiO5 crystal and then later released, but it still retained its entanglement with the unstored photon.

Holger P. Specht and co-workers at the Max Planck Institute for Quantum Optics, Garching, demonstrated a system in which a quantum bit, or qubit (a photon whose polarization states contain information), was absorbed by a single rubidium atom trapped inside an optical cavity. The rubidium atom later emitted a photon containing the original polarized information. Thus, the rubidium atom served as a quantum computer memory.

In a very different approach, Christian Ospelkaus of Leibniz University, Hannover, Ger., and colleagues used a waveguide integrated on a microchip to produce the first microwave quantum gate—that is, a logic gate for a quantum computer. Two ions were trapped just above the chip’s surface. Multiple pulses of microwave radiation entangled the two ions, which acted as a quantum gate. N. Timoney and colleagues at the University of Siegen, Ger., trapped individual ions and applied microwave pulses to them to decouple them from outside noise and thus make an undisturbed quantum processor. Such developments could aid the production of large ion-trap quantum computers in the foreseeable future.

Relativity

Results from Gravity Probe B, one of NASA’s longest-running missions, confirmed two predictions of Einstein’s general theory of relativity. It observed geodetic precession, in which the curvature of space-time around Earth induces a slight wobble in an orbiting gyroscope, and also gravitomagnetism, in which the spin of a massive object such as Earth tugs space-time in the direction of its rotation.

However, the OPERA group at the Gran Sasso National Laboratory, near Aquila, Italy, studying a beam of neutrinos generated 730 km (454 mi) away at CERN, caused a stir when they announced results that appeared to show that the particles had traveled faster than the speed of light, the fundamental limiting speed that underlies the special theory of relativity. If confirmed, this would call into question the whole basis of modern physics. The group made their results public in the hope that the experiment would be repeated independently and reasons would be identified for their unexpected finding.

Astronomy

Solar System

New discoveries about planets in the solar system provided some of the major astronomical headlines in 2011. In August scientists announced that cameras on board Mars Reconnaissance Orbiter (MRO) captured images of what appeared to be water flowing on the surface of Mars. MRO took pictures of dark streaks emerging from a slope in Newton crater and then flowing downhill. These streaks began during Martian spring and increased in length through Martian summer. The best candidate for a material that would begin melting at the right temperature was salty water. The likely presence of surviving underground water on Mars buoyed hopes that perhaps microbial life still survived there.

For information on Eclipses, Equinoxes, and Solstices, and Earth Perihelion and Aphelion in 2012, see below.

Minor planets and asteroids are among the smallest members of the solar system, along with comets and some of the moons of the major planets. On July 16 NASA’s Dawn spacecraft arrived at Vesta, the second largest main-belt asteroid. Vesta revolved around the Sun in an orbit lying between the orbits of Mars and Jupiter. During the following months the spacecraft mapped Vesta’s surface with unprecedented spatial resolution. Images showed that the asteroid has a diameter of 530 km (330 mi) and is highly pockmarked with many meteor-impact craters, particularly in its northern hemisphere. The southern hemisphere’s surface appeared to be somewhat smoother. Scientists speculated that a collision with another solar system body might have obliterated some of the southern hemisphere’s earlier craters. The most notable feature on the surface of Vesta is a large circular depression at its south pole, which is surrounded by cliffs several kilometres in height. Vesta also has a long set of ridges and grooves running along its equator. A mountain about 22 km (13 mi) high, roughly three times the height of Mt. Everest, was discovered on the southernmost part of the body. The Dawn spacecraft would continue to make a variety of scientific measurements of the properties of Vesta until July 2012, at which time it would depart for the even larger main-belt asteroid and dwarf planet Ceres.

The asteriod Vesta appears in an image taken by the Dawn spacecraft on July 24, 2011.

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In 2006 the International Astronomical Union demoted Pluto from being one of the nine major planets to one of the tens of thousands of minor planets. Nevertheless, the object continued to surprise and fascinate scientists and the public alike. In July astronomers using the Hubble Space Telescope announced the discovery of a new moon of Pluto; they also checked earlier Hubble images and found faint traces of what appeared to be the moon in images from 2006 and 2010. This brought to four the total number of moons discovered for this minor planet. The new moon, P4, is only about 13–34 km (8–21 mi) across. The three previously discovered moons—Nix, Hydra, and Charon—have diameters ranging from about 81 km (50 mi) for Hydra to about 1,200 km (750 mi) for Charon.

Stars and Extrasolar Planets

By the end of 2011, more than 700 extrasolar planets had been discovered. Of these, more than 210 planets were in multiplanet systems, a few of these somewhat similar to the solar system. On the basis of the European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS) survey, it was estimated that at least half of the stars in the Milky Way Galaxy that are similar to the Sun have planets in orbit around them. NASA’s Kepler spacecraft science team announced that it had identified over 2,000 additional planet candidates. The Kepler satellite in 2011 came closer to its goal of finding an Earth-size planet in another star’s habitable zone (the region where liquid water could survive on a planet’s surface) when it found the first Earth-size planets, Kepler-20e and Kepler-20f, which are 0.87 and 1.03 times the radius of Earth, respectively. However, with orbital periods of 6.1 and 19.6 days, respectively, they orbit too close to their star for liquid water to survive. Kepler also detected a planet orbiting in the habitable zone. The planet Kepler-22b orbits a star similar to the Sun every 290 days and has a radius 2.4 times that of Earth. Another Kepler discovery was in orbit around two stars. The planet Kepler-16b is part of an eclipsing binary star system where the stars and planet pass in front of one another periodically blocking some of the light from the telescope on board Kepler. This planet is a cold inhospitable place roughly the size of Saturn, and it orbits the two stars in about 229 days. Another planet studied by the Kepler mission, TrES-2b, is in a very close orbit around its central star and has a temperature of about 1,000 °C (1,800 °F). What was most unusual about the planet was that it reflects less than 1% of the light striking it, making its surface blacker than coal or any other natural substance found on Earth. Another remarkable discovery was Kepler-10b. It orbits so close to its star that it appears to be tidally locked to it, with one side of the planet always facing the star. The planet is about 40% larger in diameter than Earth but has a mass of about 4.6 Earth masses. This meant that the average density of the planet is about 8.8 g/cc. (Earth has a mean density of 5.5 g/cc.) This suggested that the planet consists almost entirely of rock and metal.

The first planet found to orbit two stars, Kepler-16b (artist’s conception).

T. Pyle—JPL-Caltech/NASA

The first reported candidates for black holes within the Milky Way Galaxy were objects that were detected as members of binary star systems. These objects were not seen directly in visible light. Instead, they were detected as X-ray sources, where the X-rays were thought to be radiated by disks of hot gas surrounding the purported black holes. Since the 1960s Cygnus X-1, the brightest X-ray source in the constellation Cygnus, had been the most prominent of the black hole candidates. The case for the X-ray emitter’s being a black hole, however, was somewhat circumstantial. Using rough estimates of the distance to the source and best estimates for the mass of the fairly normal companion star in the binary system, scientists concluded that the mass of the unseen star was 5–10 times the mass of the Sun. This mass was too high for the optically invisible star to be either a white dwarf or a neutron star, which led to the conclusion that it must be a black hole. In 2011, some 40 years after the system’s discovery, detailed properties of it were finally determined, leaving very little room for ambiguity. The new findings were published in a series of three papers. The first study, by M.J. Reid and collaborators from the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., used the Very Long Baseline Array of radio telescopes that were distributed around the world to determine an accurate distance to the source of approximately 6,070 light-years. This facilitated the second study, which used a variety of optical and X-ray observations to determine the mass of the black hole. The result, reported by J.A. Orosz and colleagues from San Diego State University, was that the black hole has a mass of 15 solar masses with an uncertainty of less than one solar mass. The third study, by L. Gou and collaborators from the Harvard-Smithsonian Center for Astrophysics, showed that the black hole is spinning at a rate of more than 800 rotations per second. This was very nearly the maximum rotation rate that the general theory of relativity allows for a black hole of this mass.

Galaxies and Cosmology

The beginning of 2011 saw the record broken for the most-distant astronomical object ever detected. Using the Hubble Ultra Deep Field image 09, a team led by Rychard Bouwens of the University of California, Santa Cruz, found a galaxy with a redshift of 10.3. The light from the galaxy took 13.2 billion years to arrive at Earth. This meant that it formed a mere 500 million years after the big bang.

According to the best observational evidence, the universe began with a hot dense phase that resulted in the synthesis of hydrogen and helium with only trace amounts of heavier elements. Most of the heavier elements were made much later through nuclear fusion in the interiors of stars. However, no truly primordial gas had been observed until now; all the previous detections of gas at high redshifts contained some heavier elements. In November, Michele Fumagalli and colleagues from the University of California, Santa Cruz, announced that they had observed two clouds of intergalactic gas at redshifts of 3.5 and 3.3 (originating when the universe was only about two billion years old). These clouds were truly “pristine,” containing mainly hydrogen and helium. In one cloud the team observed deuterium, an isotope of hydrogen, in the amount predicted by the best theoretical model.